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Re: glibc 2.21 - Machine maintainers, please test your machines.
- From: Chris Metcalf <cmetcalf at ezchip dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Torvald Riegel <triegel at redhat dot com>, Carlos O'Donell <carlos at redhat dot com>, GNU C Library <libc-alpha at sourceware dot org>, David Miller <davem at davemloft dot net>, Richard Henderson <rth at redhat dot com>, Mike Frysinger <vapier at gentoo dot org>, Andreas Schwab <schwab at suse dot de>, "Joseph S. Myers" <joseph at codesourcery dot com>, Kaz Kojima <kkojima at rr dot iij4u dot or dot jp>, Thomas Schwinge <thomas at codesourcery dot com>, Marcus Shawcroft <marcus dot shawcroft at linaro dot org>, Chung-Lin Tang <chunglin_tang at mentor dot com>, Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>, Andreas Krebbel <krebbel at linux dot vnet dot ibm dot com>
- Date: Sun, 25 Jan 2015 19:46:11 -0500
- Subject: Re: glibc 2.21 - Machine maintainers, please test your machines.
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On 1/25/2015 6:05 PM, H.J. Lu wrote:
On Sun, Jan 25, 2015 at 2:10 PM, Chris Metcalf <cmetcalf@ezchip.com> wrote:
On 1/25/2015 4:51 PM, H.J. Lu wrote:
This is for x86. Each target can do something like it if needed.
This does change the ABI for existing code, however, which may legitimately
have sem_t objects that are only aligned to 4-byte boundaries. Your change
is a good idea for new platforms. I suppose we could also do something
with symbol versioning to use it for existing 32-bit platforms with
64-bit atomics, too. So there seem to be multiple fixes we can consider.
It doesn't change the size, only increases alignment from 4 bytes to 8 bytes.
It may not work on all targets. It works for x32.
The size is not the issue I'm referring to; it's the alignment.
Imagine you have some code built against glibc 2.20 or older, where the
user has written:
sem_t my_sem;
Given the 4-byte alignment constraints, this could result in my_sem
being aligned to 4 bytes but not to 8 bytes. Now if we were to make the
change you propose for glibc 2.21, the user passing this semaphore to
sem_post() could be passing a pointer not aligned to 8 bytes, despite
the new headers saying that that was required. Bang, bus error -- or on x32,
presumably, worse performance even if it performs correctly.
--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com