* Potential data race in dput and __d_lookup
@ 2014-11-20 17:20 Andrey Konovalov
2014-11-20 17:41 ` Will Deacon
2014-11-20 18:05 ` Al Viro
0 siblings, 2 replies; 4+ messages in thread
From: Andrey Konovalov @ 2014-11-20 17:20 UTC (permalink / raw)
To: Ingo Molnar, Will Deacon, Peter Zijlstra, Davidlohr Bueso,
Alexander Viro, linux-fsdevel, linux-kernel
Cc: Dmitry Vyukov, Kostya Serebryany, ktsan
Hi
We are working on a dynamic data race detector for Linux kernel called
KernelThreadSanitizer (ktsan)
(https://code.google.com/p/thread-sanitizer/wiki/ThreadSanitizerForKernel).
Here is a report we got while running ktsan (upstream revision
fc14f9c1272f62c3e8d01300f52467c0d9af50f9, Linux 3.18-rc5):
==================================================================
ThreadSanitizer: data-race in lockref_put_or_lock
Read of size 8 by thread T575 (K814):
[<ffffffff8152067f>] lockref_put_or_lock+0x1f/0xe0 /lib/lockref.c:122
[<ffffffff8126965e>] dput+0x2e/0x2b0 /fs/dcache.c:626
[< inlined >] link_path_walk+0xddd/0x1d40 path_to_nameidata
/fs/namei.c:677
[< inlined >] link_path_walk+0xddd/0x1d40 walk_component
/fs/namei.c:1571
[<ffffffff81257d7d>] link_path_walk+0xddd/0x1d40 /fs/namei.c:1805
[<ffffffff8125e344>] path_openat+0xe4/0xb10 /fs/namei.c:3206
[<ffffffff81260911>] do_filp_open+0x51/0xd0 /fs/namei.c:3259
[<ffffffff81242003>] do_sys_open+0x183/0x2d0 /fs/open.c:998
[< inlined >] SyS_open+0x35/0x50 SYSC_open /fs/open.c:1016
[<ffffffff81242185>] SyS_open+0x35/0x50 /fs/open.c:1011
[<ffffffff81e39fe9>] system_call_fastpath+0x12/0x17
/arch/x86/kernel/entry_64.S:422
DBG: cpu = ffffe8ffffc010b0
Previous write of size 4 by thread T574 (K813):
[<ffffffff8126e22f>] __d_lookup+0x27f/0x2d0 /fs/dcache.c:2185
[<ffffffff812543c9>] lookup_fast+0x299/0x5a0 /fs/namei.c:1427
[< inlined >] link_path_walk+0x25c/0x1d40 walk_component
/fs/namei.c:1546
[<ffffffff812571fc>] link_path_walk+0x25c/0x1d40 /fs/namei.c:1805
[<ffffffff8125e344>] path_openat+0xe4/0xb10 /fs/namei.c:3206
[<ffffffff81260911>] do_filp_open+0x51/0xd0 /fs/namei.c:3259
[<ffffffff81242003>] do_sys_open+0x183/0x2d0 /fs/open.c:998
[< inlined >] SyS_open+0x35/0x50 SYSC_open /fs/open.c:1016
[<ffffffff81242185>] SyS_open+0x35/0x50 /fs/open.c:1011
[<ffffffff81e39fe9>] system_call_fastpath+0x12/0x17
/arch/x86/kernel/entry_64.S:422
DBG: cpu = 0
DBG: addr: ffff8801148e91f0
DBG: first offset: 4, second offset: 0
DBG: T575 clock: {T575: 27630, T574: 25486}
DBG: T574 clock: {T574: 25539}
==================================================================
It seems that one thread increments 'dentry->d_lockref.count', while
other does 'lockref_put_or_lock(&dentry->d_lockref)' without any
synchronization.
Could you confirm if this is a real race?
Thank you.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Potential data race in dput and __d_lookup
2014-11-20 17:20 Potential data race in dput and __d_lookup Andrey Konovalov
@ 2014-11-20 17:41 ` Will Deacon
2014-11-20 18:11 ` Al Viro
2014-11-20 18:05 ` Al Viro
1 sibling, 1 reply; 4+ messages in thread
From: Will Deacon @ 2014-11-20 17:41 UTC (permalink / raw)
To: Andrey Konovalov
Cc: Ingo Molnar, Peter Zijlstra, Davidlohr Bueso, Alexander Viro,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Dmitry Vyukov, Kostya Serebryany, ktsan@googlegroups.com
On Thu, Nov 20, 2014 at 05:20:14PM +0000, Andrey Konovalov wrote:
> Hi
>
> We are working on a dynamic data race detector for Linux kernel called
> KernelThreadSanitizer (ktsan)
> (https://code.google.com/p/thread-sanitizer/wiki/ThreadSanitizerForKernel).
>
> Here is a report we got while running ktsan (upstream revision
> fc14f9c1272f62c3e8d01300f52467c0d9af50f9, Linux 3.18-rc5):
>
> ==================================================================
> ThreadSanitizer: data-race in lockref_put_or_lock
>
> Read of size 8 by thread T575 (K814):
> [<ffffffff8152067f>] lockref_put_or_lock+0x1f/0xe0 /lib/lockref.c:122
> [<ffffffff8126965e>] dput+0x2e/0x2b0 /fs/dcache.c:626
> [< inlined >] link_path_walk+0xddd/0x1d40 path_to_nameidata
> /fs/namei.c:677
> [< inlined >] link_path_walk+0xddd/0x1d40 walk_component
> /fs/namei.c:1571
> [<ffffffff81257d7d>] link_path_walk+0xddd/0x1d40 /fs/namei.c:1805
> [<ffffffff8125e344>] path_openat+0xe4/0xb10 /fs/namei.c:3206
> [<ffffffff81260911>] do_filp_open+0x51/0xd0 /fs/namei.c:3259
> [<ffffffff81242003>] do_sys_open+0x183/0x2d0 /fs/open.c:998
> [< inlined >] SyS_open+0x35/0x50 SYSC_open /fs/open.c:1016
> [<ffffffff81242185>] SyS_open+0x35/0x50 /fs/open.c:1011
> [<ffffffff81e39fe9>] system_call_fastpath+0x12/0x17
> /arch/x86/kernel/entry_64.S:422
> DBG: cpu = ffffe8ffffc010b0
>
> Previous write of size 4 by thread T574 (K813):
> [<ffffffff8126e22f>] __d_lookup+0x27f/0x2d0 /fs/dcache.c:2185
> [<ffffffff812543c9>] lookup_fast+0x299/0x5a0 /fs/namei.c:1427
> [< inlined >] link_path_walk+0x25c/0x1d40 walk_component
> /fs/namei.c:1546
> [<ffffffff812571fc>] link_path_walk+0x25c/0x1d40 /fs/namei.c:1805
> [<ffffffff8125e344>] path_openat+0xe4/0xb10 /fs/namei.c:3206
> [<ffffffff81260911>] do_filp_open+0x51/0xd0 /fs/namei.c:3259
> [<ffffffff81242003>] do_sys_open+0x183/0x2d0 /fs/open.c:998
> [< inlined >] SyS_open+0x35/0x50 SYSC_open /fs/open.c:1016
> [<ffffffff81242185>] SyS_open+0x35/0x50 /fs/open.c:1011
> [<ffffffff81e39fe9>] system_call_fastpath+0x12/0x17
> /arch/x86/kernel/entry_64.S:422
> DBG: cpu = 0
>
> DBG: addr: ffff8801148e91f0
> DBG: first offset: 4, second offset: 0
> DBG: T575 clock: {T575: 27630, T574: 25486}
> DBG: T574 clock: {T574: 25539}
> ==================================================================
>
> It seems that one thread increments 'dentry->d_lockref.count', while
> other does 'lockref_put_or_lock(&dentry->d_lockref)' without any
> synchronization.
>
> Could you confirm if this is a real race?
I think it should be fine. d_lock is #defined as d_lockref.lock, and the
whole way the lockref works is that you can either cmpxchg the lock and the
counter, or take the lock and do what you like.
So in this case, the increment is done with the lock held, which will
cause a competing lockref_put_or_lock to fail on the cmpxchg path.
Will
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Potential data race in dput and __d_lookup
2014-11-20 17:20 Potential data race in dput and __d_lookup Andrey Konovalov
2014-11-20 17:41 ` Will Deacon
@ 2014-11-20 18:05 ` Al Viro
1 sibling, 0 replies; 4+ messages in thread
From: Al Viro @ 2014-11-20 18:05 UTC (permalink / raw)
To: Andrey Konovalov
Cc: Ingo Molnar, Will Deacon, Peter Zijlstra, Davidlohr Bueso,
linux-fsdevel, linux-kernel, Dmitry Vyukov, Kostya Serebryany,
ktsan
On Thu, Nov 20, 2014 at 09:20:14PM +0400, Andrey Konovalov wrote:
> It seems that one thread increments 'dentry->d_lockref.count', while
> other does 'lockref_put_or_lock(&dentry->d_lockref)' without any
> synchronization.
>
> Could you confirm if this is a real race?
I can not. Increment in __d_lookup() is under ->d_lockref.lock (aka
->d_lock) and lockref_put_or_lock() is atomic wrt that.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Potential data race in dput and __d_lookup
2014-11-20 17:41 ` Will Deacon
@ 2014-11-20 18:11 ` Al Viro
0 siblings, 0 replies; 4+ messages in thread
From: Al Viro @ 2014-11-20 18:11 UTC (permalink / raw)
To: Will Deacon
Cc: Andrey Konovalov, Ingo Molnar, Peter Zijlstra, Davidlohr Bueso,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Dmitry Vyukov, Kostya Serebryany, ktsan@googlegroups.com
On Thu, Nov 20, 2014 at 05:41:01PM +0000, Will Deacon wrote:
> I think it should be fine. d_lock is #defined as d_lockref.lock, and the
> whole way the lockref works is that you can either cmpxchg the lock and the
> counter
... both by a single operation, that is.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2014-11-20 17:20 Potential data race in dput and __d_lookup Andrey Konovalov
2014-11-20 17:41 ` Will Deacon
2014-11-20 18:11 ` Al Viro
2014-11-20 18:05 ` Al Viro
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