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Re: [PATCH v2] nptl: optimize cancelstate and canceltype changing
- From: Alexander Fyodorov <halcy at yandex dot ru>
- To: Rich Felker <dalias at libc dot org>
- Cc: "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>, Carlos O'Donell <carlos at redhat dot com>, David Miller <davem at davemloft dot net>
- Date: Sun, 15 Jun 2014 20:48:35 +0400
- Subject: Re: [PATCH v2] nptl: optimize cancelstate and canceltype changing
- Authentication-results: sourceware.org; auth=none
- References: <180951402831608 at web26g dot yandex dot ru> <20140615151751 dot GQ179 at brightrain dot aerifal dot cx>
15.06.2014, 19:17, "Rich Felker" <email@example.com>:
> On Sun, Jun 15, 2014 at 03:26:48PM +0400, Alexander Fyodorov wrote:
>> Changes since v1:
>> 1) Added a benchtest for pthread_setcanceltype().
>> 2) Defined atomic_write() macro for old SPARC processors.
>> atomic_write() macro is needed because on some architectures there
>> is no hardware support for atomic compare-and-swap, and it is
>> implemented via spinlock. And writing to variables that are accessed
>> with such compare-and-swap must be done under the same spinlock to
>> avoid the race leading to the lost write.
> How is this handled for atomic CAS in shared memory? IMO the only way
> to do this with a spinlock is having it be accessible by all
> processes, which would allow any process to deadlock all other
> processes maliciously. The only solution seems to be requiring a
> syscall for CAS, or (preferably) doing something like the ARM kuser
> helper for CAS and refusing to do SMP on such machines.
Good catch, I forgot about shared memory. Currently this macro is only used for threads descriptors which are not allocated in shared memory. So just adding an explanatory comment before atomic_write() should be the way to go I think, until the need arises to use it for shared memory.