linux-kernel - Re: Nonblocking call may block in a mutex? Nonblocking call after poll may fail?

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Message-Id: <200708311532.26376.vda.linux@googlemail.com>
Date:	Fri, 31 Aug 2007 15:32:26 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	"anon... anon.al" <anon.asdf@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Nonblocking call may block in a mutex? Nonblocking call after poll may fail?

On Friday 31 August 2007 13:13, anon... anon.al wrote:
> Hi!
>
> This is a driver-related question on non-blocking writes and poll.
>
> Setup:
> there is a single output-buffer (in kernel-space) of 24 bytes for
> writes from all processes A, B, and C: each process is restricted to
> use at most 8 bytes: 8*3 = 24
> (until that data is handled (interrupt-handler...))
>
> Question:
> If this output-buffer has "4-bytes space remaining for process A",
> then a non-blocking write of process A could still encounter a locked
> mutex, if process B is busy writing to the output-buffer.
>
> Should process A now block/sleep until that mutex is free and it can
> access the output-buffer (and it's 4 bytes space)?
>
> What about a non-blocking (write-) poll of process A: if the poll call
> succeeds (the output buffer has space remaining for process A), and
> process A now performs a non-blocking write: what happens if A
> encounters a blocked mutex, since process B is busy writing to the
> output-buffer.
> a) Should A block until the mutex is available?

If mutex cannot be locked by B indefinitely, yes.
If it can be locked indefinitely, then obviosly no.

> b) Should A return -EAGAIN, even though the poll call succeeded?

Succeeding poll is no guarantee against getting EAGAIN.
When poll succeeds, it means "you _maybe_ can write now".

> c) Should it be impossible for this to happen! i.e. -> should process
> A already "have" the mutex in question, when the poll call succeeds
> (thus preventing B from writing to the output buffer)

No. Kernel cannot know whether A will do the write at all.

> For c) What if process A "has" the mutex, but never does the
> non-blocking write. Then no process can write, since the mutex is held
> by process A...

Exactly. (c) would be kernel bug.
--
vda
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