From: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
To: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: 'GNU C Library' <libc-alpha@sourceware.org>
Subject: Re: [PATCH] Avoid RMW of flags2 outside lock (BZ #27842)
Date: Fri, 20 May 2022 10:34:58 +0000 [thread overview]
Message-ID: <DB6PR0801MB18797FEE9F5287339C60F3DD83D39@DB6PR0801MB1879.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <eaa94d84-6cbc-2825-4029-9a7d692a1ecb@linaro.org>
Hi Adhemerval,
> I am trying to see why exactly we need to disable single-thread optimization
> on flockfile, since there is no FILE operation that takes a callback where
> pthread_create might be called beween _IO_acquire_lock. Can't we just remove
> the _IO_FLAGS2_NEED_LOCK set on flockfile?
Yes we could. I don't believe you could actually notice the difference - recursion
or starting a thread before funlockfile would work fine.
>> Right so you mean moving NEED_LOCK bit into the lock variable?
>
> Yes, and making the lock smaller on linux (just a word plus the recursive counter).
OK, it will be interesting to see how that works out. Any lock speedup will be
good, however I guess that recursive locks will remain quite expensive. By design
the single-thread optimization does not need to support things like trylock,
recursion or starting a new thread, and that is a huge advantage.
> You mean as it is now? It is a real bug since various functions update flags2
> behind a lock, so it is possible for this RMW to cause corruption (but only if
> you are already multithreaded, which the update is pointless anyway and we
> can just skip it).
> I don't think it would be possible to corrupt because once pthread_create
> is called, _IO_FLAGS2_NEED_LOCK will be always set (so RMW won't see a
> __flags2 without _IO_FLAGS2_NEED_LOCK being set).
The issue is not corruption of _IO_FLAGS2_NEED_LOCK but of a different flag.
Several IO functions may set or clear bits in flag2. So we could get a race
between RMW sequences setting/clearing different bits - if there is any overlap
one of the updates may be lost.
Cheers,
Wilco
prev parent reply other threads:[~2022-05-20 10:35 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-19 15:10 Wilco Dijkstra
2022-05-19 15:25 ` Andreas Schwab
2022-05-19 15:47 ` Adhemerval Zanella
2022-05-19 16:26 ` Wilco Dijkstra
2022-05-19 16:57 ` Adhemerval Zanella
2022-05-20 10:34 ` Wilco Dijkstra [this message]
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