Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 13 Nov 2019 08:40:41 -0800 | Subject | Re: [PATCH v2] buffer: Fix I/O error due to ARM read-after-read hazard |
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On Wed, Nov 13, 2019 at 8:36 AM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > test_bit() is a very unfortunate interface, in that we actually use it > in some situations where we _really_ would want to merge reads (not > split them, but merge them). There are several cases where we do > constant test-bits on the same word, and don't care about ordering. > Things like thread flags etc.
Side note: test_bit() really isn't good for locking in the first place. The fact that the buffer heads use it for that is very non-optimal indeed.
Particularly for testing something like "is this buffer uptodate", it should be a "smp_load_acquire()", not a test_bit(). And READ_ONCE() doesn't really help.
So in many ways it would be much better to make the buffer head stuff use proper ordered accesses. But I suspect nobody is going to ever want to go through that pain for a legacy thing, so the papering it over with READ_ONCE() and a ugly ARM hw erratum hack is probably the best we'll do..
Linus
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