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2019-06-07lockref: Limit number of cmpxchg loop retriesJan Glauber
The lockref cmpxchg loop is unbound as long as the spinlock is not taken. Depending on the hardware implementation of compare-and-swap a high number of loop retries might happen. Add an upper bound to the loop to force the fallback to spinlocks after some time. A retry value of 100 should not impact any hardware that does not have this issue. With the retry limit the performance of an open-close testcase improved between 60-70% on ThunderX2. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Jan Glauber <jglauber@marvell.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2018-04-12lockref: Add lockref_put_not_zeroAndreas Gruenbacher
Put a lockref unless the lockref is dead or its count would become zero. This is the same as lockref_put_or_lock except that the lock is never left held. Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com> Signed-off-by: Bob Peterson <rpeterso@redhat.com>
2017-11-02License cleanup: add SPDX GPL-2.0 license identifier to files with no licenseGreg Kroah-Hartman
Many source files in the tree are missing licensing information, which makes it harder for compliance tools to determine the correct license. By default all files without license information are under the default license of the kernel, which is GPL version 2. Update the files which contain no license information with the 'GPL-2.0' SPDX license identifier. The SPDX identifier is a legally binding shorthand, which can be used instead of the full boiler plate text. This patch is based on work done by Thomas Gleixner and Kate Stewart and Philippe Ombredanne. How this work was done: Patches were generated and checked against linux-4.14-rc6 for a subset of the use cases: - file had no licensing information it it. - file was a */uapi/* one with no licensing information in it, - file was a */uapi/* one with existing licensing information, Further patches will be generated in subsequent months to fix up cases where non-standard license headers were used, and references to license had to be inferred by heuristics based on keywords. The analysis to determine which SPDX License Identifier to be applied to a file was done in a spreadsheet of side by side results from of the output of two independent scanners (ScanCode & Windriver) producing SPDX tag:value files created by Philippe Ombredanne. Philippe prepared the base worksheet, and did an initial spot review of a few 1000 files. The 4.13 kernel was the starting point of the analysis with 60,537 files assessed. Kate Stewart did a file by file comparison of the scanner results in the spreadsheet to determine which SPDX license identifier(s) to be applied to the file. She confirmed any determination that was not immediately clear with lawyers working with the Linux Foundation. Criteria used to select files for SPDX license identifier tagging was: - Files considered eligible had to be source code files. - Make and config files were included as candidates if they contained >5 lines of source - File already had some variant of a license header in it (even if <5 lines). All documentation files were explicitly excluded. The following heuristics were used to determine which SPDX license identifiers to apply. - when both scanners couldn't find any license traces, file was considered to have no license information in it, and the top level COPYING file license applied. For non */uapi/* files that summary was: SPDX license identifier # files ---------------------------------------------------|------- GPL-2.0 11139 and resulted in the first patch in this series. If that file was a */uapi/* path one, it was "GPL-2.0 WITH Linux-syscall-note" otherwise it was "GPL-2.0". Results of that was: SPDX license identifier # files ---------------------------------------------------|------- GPL-2.0 WITH Linux-syscall-note 930 and resulted in the second patch in this series. - if a file had some form of licensing information in it, and was one of the */uapi/* ones, it was denoted with the Linux-syscall-note if any GPL family license was found in the file or had no licensing in it (per prior point). Results summary: SPDX license identifier # files ---------------------------------------------------|------ GPL-2.0 WITH Linux-syscall-note 270 GPL-2.0+ WITH Linux-syscall-note 169 ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause) 21 ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause) 17 LGPL-2.1+ WITH Linux-syscall-note 15 GPL-1.0+ WITH Linux-syscall-note 14 ((GPL-2.0+ WITH Linux-syscall-note) OR BSD-3-Clause) 5 LGPL-2.0+ WITH Linux-syscall-note 4 LGPL-2.1 WITH Linux-syscall-note 3 ((GPL-2.0 WITH Linux-syscall-note) OR MIT) 3 ((GPL-2.0 WITH Linux-syscall-note) AND MIT) 1 and that resulted in the third patch in this series. - when the two scanners agreed on the detected license(s), that became the concluded license(s). - when there was disagreement between the two scanners (one detected a license but the other didn't, or they both detected different licenses) a manual inspection of the file occurred. - In most cases a manual inspection of the information in the file resulted in a clear resolution of the license that should apply (and which scanner probably needed to revisit its heuristics). - When it was not immediately clear, the license identifier was confirmed with lawyers working with the Linux Foundation. - If there was any question as to the appropriate license identifier, the file was flagged for further research and to be revisited later in time. In total, over 70 hours of logged manual review was done on the spreadsheet to determine the SPDX license identifiers to apply to the source files by Kate, Philippe, Thomas and, in some cases, confirmation by lawyers working with the Linux Foundation. Kate also obtained a third independent scan of the 4.13 code base from FOSSology, and compared selected files where the other two scanners disagreed against that SPDX file, to see if there was new insights. The Windriver scanner is based on an older version of FOSSology in part, so they are related. Thomas did random spot checks in about 500 files from the spreadsheets for the uapi headers and agreed with SPDX license identifier in the files he inspected. For the non-uapi files Thomas did random spot checks in about 15000 files. In initial set of patches against 4.14-rc6, 3 files were found to have copy/paste license identifier errors, and have been fixed to reflect the correct identifier. Additionally Philippe spent 10 hours this week doing a detailed manual inspection and review of the 12,461 patched files from the initial patch version early this week with: - a full scancode scan run, collecting the matched texts, detected license ids and scores - reviewing anything where there was a license detected (about 500+ files) to ensure that the applied SPDX license was correct - reviewing anything where there was no detection but the patch license was not GPL-2.0 WITH Linux-syscall-note to ensure that the applied SPDX license was correct This produced a worksheet with 20 files needing minor correction. This worksheet was then exported into 3 different .csv files for the different types of files to be modified. These .csv files were then reviewed by Greg. Thomas wrote a script to parse the csv files and add the proper SPDX tag to the file, in the format that the file expected. This script was further refined by Greg based on the output to detect more types of files automatically and to distinguish between header and source .c files (which need different comment types.) Finally Greg ran the script using the .csv files to generate the patches. Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org> Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2016-11-16locking/core: Remove cpu_relax_lowlatency() usersChristian Borntraeger
With the s390 special case of a yielding cpu_relax() implementation gone, we can now remove all users of cpu_relax_lowlatency() and replace them with cpu_relax(). Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Nicholas Piggin <npiggin@gmail.com> Cc: Noam Camus <noamc@ezchip.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Russell King <linux@armlinux.org.uk> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will.deacon@arm.com> Cc: linuxppc-dev@lists.ozlabs.org Cc: virtualization@lists.linux-foundation.org Cc: xen-devel@lists.xenproject.org Link: http://lkml.kernel.org/r/1477386195-32736-5-git-send-email-borntraeger@de.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-08-12locking/lockref: Remove homebrew cmpxchg64_relaxed() macro definitionWill Deacon
cmpxchg64_relaxed() is now defined by linux/atomic.h, so we can remove our local definition from the lockref code. Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Waiman.Long@hp.com Cc: paulmck@linux.vnet.ibm.com Link: http://lkml.kernel.org/r/1438880084-18856-5-git-send-email-will.deacon@arm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-02-24locking: Remove ACCESS_ONCE() usageDavidlohr Bueso
With the new standardized functions, we can replace all ACCESS_ONCE() calls across relevant locking - this includes lockref and seqlock while at it. ACCESS_ONCE() does not work reliably on non-scalar types. For example gcc 4.6 and 4.7 might remove the volatile tag for such accesses during the SRA (scalar replacement of aggregates) step: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 Update the new calls regardless of if it is a scalar type, this is cleaner than having three alternatives. Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/1424662301.6539.18.camel@stgolabs.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-01-25dcache: let the dentry count go down to zero without taking d_lockLinus Torvalds
We can be more aggressive about this, if we are clever and careful. This is subtle. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2014-07-17arch, locking: Ciao arch_mutex_cpu_relax()Davidlohr Bueso
The arch_mutex_cpu_relax() function, introduced by 34b133f, is hacky and ugly. It was added a few years ago to address the fact that common cpu_relax() calls include yielding on s390, and thus impact the optimistic spinning functionality of mutexes. Nowadays we use this function well beyond mutexes: rwsem, qrwlock, mcs and lockref. Since the macro that defines the call is in the mutex header, any users must include mutex.h and the naming is misleading as well. This patch (i) renames the call to cpu_relax_lowlatency ("relax, but only if you can do it with very low latency") and (ii) defines it in each arch's asm/processor.h local header, just like for regular cpu_relax functions. On all archs, except s390, cpu_relax_lowlatency is simply cpu_relax, and thus we can take it out of mutex.h. While this can seem redundant, I believe it is a good choice as it allows us to move out arch specific logic from generic locking primitives and enables future(?) archs to transparently define it, similarly to System Z. Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Anton Blanchard <anton@samba.org> Cc: Aurelien Jacquiot <a-jacquiot@ti.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Bharat Bhushan <r65777@freescale.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Chen Liqin <liqin.linux@gmail.com> Cc: Chris Metcalf <cmetcalf@tilera.com> Cc: Christian Borntraeger <borntraeger@de.ibm.com> Cc: Chris Zankel <chris@zankel.net> Cc: David Howells <dhowells@redhat.com> Cc: David S. Miller <davem@davemloft.net> Cc: Deepthi Dharwar <deepthi@linux.vnet.ibm.com> Cc: Dominik Dingel <dingel@linux.vnet.ibm.com> Cc: Fenghua Yu <fenghua.yu@intel.com> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Guan Xuetao <gxt@mprc.pku.edu.cn> Cc: Haavard Skinnemoen <hskinnemoen@gmail.com> Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Helge Deller <deller@gmx.de> Cc: Hirokazu Takata <takata@linux-m32r.org> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: James E.J. Bottomley <jejb@parisc-linux.org> Cc: James Hogan <james.hogan@imgtec.com> Cc: Jason Wang <jasowang@redhat.com> Cc: Jesper Nilsson <jesper.nilsson@axis.com> Cc: Joe Perches <joe@perches.com> Cc: Jonas Bonn <jonas@southpole.se> Cc: Joseph Myers <joseph@codesourcery.com> Cc: Kees Cook <keescook@chromium.org> Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com> Cc: Lennox Wu <lennox.wu@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mark Salter <msalter@redhat.com> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Matt Turner <mattst88@gmail.com> Cc: Max Filippov <jcmvbkbc@gmail.com> Cc: Michael Neuling <mikey@neuling.org> Cc: Michal Simek <monstr@monstr.eu> Cc: Mikael Starvik <starvik@axis.com> Cc: Nicolas Pitre <nico@linaro.org> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Paul Burton <paul.burton@imgtec.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Paul Mackerras <paulus@samba.org> Cc: Qais Yousef <qais.yousef@imgtec.com> Cc: Qiaowei Ren <qiaowei.ren@intel.com> Cc: Rafael Wysocki <rafael.j.wysocki@intel.com> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Richard Henderson <rth@twiddle.net> Cc: Richard Kuo <rkuo@codeaurora.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Steven Miao <realmz6@gmail.com> Cc: Steven Rostedt <srostedt@redhat.com> Cc: Stratos Karafotis <stratosk@semaphore.gr> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Vasily Kulikov <segoon@openwall.com> Cc: Vineet Gupta <vgupta@synopsys.com> Cc: Vineet Gupta <Vineet.Gupta1@synopsys.com> Cc: Waiman Long <Waiman.Long@hp.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Wolfram Sang <wsa@the-dreams.de> Cc: adi-buildroot-devel@lists.sourceforge.net Cc: linux390@de.ibm.com Cc: linux-alpha@vger.kernel.org Cc: linux-am33-list@redhat.com Cc: linux-arm-kernel@lists.infradead.org Cc: linux-c6x-dev@linux-c6x.org Cc: linux-cris-kernel@axis.com Cc: linux-hexagon@vger.kernel.org Cc: linux-ia64@vger.kernel.org Cc: linux@lists.openrisc.net Cc: linux-m32r-ja@ml.linux-m32r.org Cc: linux-m32r@ml.linux-m32r.org Cc: linux-m68k@lists.linux-m68k.org Cc: linux-metag@vger.kernel.org Cc: linux-mips@linux-mips.org Cc: linux-parisc@vger.kernel.org Cc: linuxppc-dev@lists.ozlabs.org Cc: linux-s390@vger.kernel.org Cc: linux-sh@vger.kernel.org Cc: linux-xtensa@linux-xtensa.org Cc: sparclinux@vger.kernel.org Link: http://lkml.kernel.org/r/1404079773.2619.4.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-11-27lockref: include mutex.h rather than reinvent arch_mutex_cpu_relaxWill Deacon
arch_mutex_cpu_relax is already conditionally defined in mutex.h, so simply include that header rather than replicate the code here. Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-11-15lockref: use BLOATED_SPINLOCKS to avoid explicit config dependenciesPeter Zijlstra
Avoid the fragile Kconfig construct guestimating spinlock_t sizes; use a friendly compile-time test to determine this. [kirill.shutemov@linux.intel.com: drop CONFIG_CMPXCHG_LOCKREF] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-11-11Merge tag 'gfs2-merge-window' of ↵Linus Torvalds
git://git.kernel.org/pub/scm/linux/kernel/git/steve/gfs2-3.0-nmw Pull gfs2 updates from Steven Whitehouse: "The main feature of interest this time is quota updates. There are some clean ups and some patches to use the new generic lru list code. There is still plenty of scope for some further changes in due course - faster lookups of quota structures is very much on the todo list. Also, a start has been made towards the more tricky issue of using the generic lru code with glocks, but that will have to be completed in a subsequent merge window. The other, more minor feature, is that there have been a number of performance patches which relate to block allocation. In particular they will improve performance when the disk is nearly full" * tag 'gfs2-merge-window' of git://git.kernel.org/pub/scm/linux/kernel/git/steve/gfs2-3.0-nmw: GFS2: Use generic list_lru for quota GFS2: Rename quota qd_lru_lock qd_lock GFS2: Use reflink for quota data cache GFS2: Use lockref for glocks GFS2: Protect quota sync generation GFS2: Inline qd_trylock into gfs2_quota_unlock GFS2: Make two similar quota code fragments into a function GFS2: Remove obsolete quota tunable GFS2: Move gfs2_icbit_munge into quota.c GFS2: Speed up starting point selection for block allocation GFS2: Add allocation parameters structure GFS2: Clean up reservation removal GFS2: fix dentry leaks GFS2: new function gfs2_rbm_incr GFS2: Introduce rbm field bii GFS2: Do not reset flags on active reservations GFS2: introduce bi_blocks for optimization GFS2: optimize rbm_from_block wrt bi_start GFS2: d_splice_alias() can't return error
2013-10-15GFS2: Use lockref for glocksSteven Whitehouse
Currently glocks have an atomic reference count and also a spinlock which covers various internal fields, such as the state. This intent of this patch is to replace the spinlock and the atomic reference count with a lockref structure. This contains a spinlock which we can continue to use as before, and a reference counter which is used in conjuction with the spinlock to replace the previous atomic counter. As a result of this there are some new rules for reference counting on glocks. We need to distinguish between reference count changes under gl_spin (which are now just increment or decrement of the new counter, provided the count cannot hit zero) and those which are outside of gl_spin, but which now take gl_spin internally. The conversion is relatively straight forward. There is probably some further clean up which can be done, but the priority at this stage is to make the change in as simple a manner as possible. A consequence of this change is that the reference count is being decoupled from the lru list processing. This should allow future adoption of the lru_list code with glocks in due course. The reason for using the "dead" state and not just relying on 0 being the "invalid state" is so that in due course 0 ref counts can be allowable. The intent is to eventually be able to remove the ref count changes which are currently hidden away in state_change(). Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
2013-09-28lockref: use arch_mutex_cpu_relax() in CMPXCHG_LOOP()Heiko Carstens
Make use of arch_mutex_cpu_relax() so architectures can override the default cpu_relax() semantics. This is especially useful for s390, where cpu_relax() means that we yield() the current (virtual) cpu and therefore is very expensive, and would contradict the whole purpose of the lockless cmpxchg loop. Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
2013-09-27lockref: allow relaxed cmpxchg64 variant for lockless updatesWill Deacon
The 64-bit cmpxchg operation on the lockref is ordered by virtue of hazarding between the cmpxchg operation and the reference count manipulation. On weakly ordered memory architectures (such as ARM), it can be of great benefit to omit the barrier instructions where they are not needed. This patch moves the lockless lockref code over to a cmpxchg64_relaxed operation, which doesn't provide barrier semantics. If the operation isn't defined, we simply #define it as the usual 64-bit cmpxchg macro. Cc: Waiman Long <Waiman.Long@hp.com> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-09-20lockref: use cmpxchg64 explicitly for lockless updatesWill Deacon
The cmpxchg() function tends not to support 64-bit arguments on 32-bit architectures. This could be either due to use of unsigned long arguments (like on ARM) or lack of instruction support (cmpxchgq on x86). However, these architectures may implement a specific cmpxchg64() function to provide 64-bit cmpxchg support instead. Since the lockref code requires a 64-bit cmpxchg and relies on the architecture selecting ARCH_USE_CMPXCHG_LOCKREF, move to using cmpxchg64 instead of cmpxchg and allow 32-bit architectures to make use of the lockless lockref implementation. Cc: Waiman Long <Waiman.Long@hp.com> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-09-07lockref: add ability to mark lockrefs "dead"Linus Torvalds
The only actual current lockref user (dcache) uses zero reference counts even for perfectly live dentries, because it's a cache: there may not be any users, but that doesn't mean that we want to throw away the dentry. At the same time, the dentry cache does have a notion of a truly "dead" dentry that we must not even increment the reference count of, because we have pruned it and it is not valid. Currently that distinction is not visible in the lockref itself, and the dentry cache validation uses "lockref_get_or_lock()" to either get a new reference to a dentry that already had existing references (and thus cannot be dead), or get the dentry lock so that we can then verify the dentry and increment the reference count under the lock if that verification was successful. That's all somewhat complicated. This adds the concept of being "dead" to the lockref itself, by simply using a count that is negative. This allows a usage scenario where we can increment the refcount of a dentry without having to validate it, and pushing the special "we killed it" case into the lockref code. The dentry code itself doesn't actually use this yet, and it's probably too late in the merge window to do that code (the dentry_kill() code with its "should I decrement the count" logic really is pretty complex code), but let's introduce the concept at the lockref level now. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-09-07lockref: fix docbook argument namesLinus Torvalds
The code got rewritten, but the comments got copied as-is from older versions, and as a result the argument name in the comment didn't actually match the code any more. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-09-03lockref: Relax in cmpxchg loopLuck, Tony
While we are likley to succeed and break out of this loop, it isn't guaranteed. We should be power and thread friendly if we do have to go around for a second (or third, or more) attempt. Signed-off-by: Tony Luck <tony.luck@intel.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-09-02lockref: implement lockless reference count updates using cmpxchg()Linus Torvalds
Instead of taking the spinlock, the lockless versions atomically check that the lock is not taken, and do the reference count update using a cmpxchg() loop. This is semantically identical to doing the reference count update protected by the lock, but avoids the "wait for lock" contention that you get when accesses to the reference count are contended. Note that a "lockref" is absolutely _not_ equivalent to an atomic_t. Even when the lockref reference counts are updated atomically with cmpxchg, the fact that they also verify the state of the spinlock means that the lockless updates can never happen while somebody else holds the spinlock. So while "lockref_put_or_lock()" looks a lot like just another name for "atomic_dec_and_lock()", and both optimize to lockless updates, they are fundamentally different: the decrement done by atomic_dec_and_lock() is truly independent of any lock (as long as it doesn't decrement to zero), so a locked region can still see the count change. The lockref structure, in contrast, really is a *locked* reference count. If you hold the spinlock, the reference count will be stable and you can modify the reference count without using atomics, because even the lockless updates will see and respect the state of the lock. In order to enable the cmpxchg lockless code, the architecture needs to do three things: (1) Make sure that the "arch_spinlock_t" and an "unsigned int" can fit in an aligned u64, and have a "cmpxchg()" implementation that works on such a u64 data type. (2) define a helper function to test for a spinlock being unlocked ("arch_spin_value_unlocked()") (3) select the "ARCH_USE_CMPXCHG_LOCKREF" config variable in its Kconfig file. This enables it for x86-64 (but not 32-bit, we'd need to make sure cmpxchg() turns into the proper cmpxchg8b in order to enable it for 32-bit mode). Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-09-02lockref: uninline lockref helper functionsLinus Torvalds
They aren't very good to inline, since they already call external functions (the spinlock code), and we're going to create rather more complicated versions of them that can do the reference count updates locklessly. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>