On 3 Apr 2018, at 23:22, Huang, Ying wrote: > From: Huang Ying <[email protected]> > > mmap_sem will be read locked when calling follow_pmd_mask(). But this > cannot prevent PMD from being changed for all cases when PTL is > unlocked, for example, from pmd_trans_huge() to pmd_none() via > MADV_DONTNEED. So it is possible for the pmd_present() check in > follow_pmd_mask() encounter a none PMD. This may cause incorrect > VM_BUG_ON() or infinite loop. Fixed this via reading PMD entry again > but only once and checking the local variable and pmd_none() in the > retry loop. > > As Kirill pointed out, with PTL unlocked, the *pmd may be changed > under us, so read it directly again and again may incur weird bugs. > So although using *pmd directly other than pmd_present() checking may > be safe, it is still better to replace them to read *pmd once and > check the local variable for multiple times.
I see you point there. The patch wants to provide a consistent value
for all race checks. Specifically, this patch is trying to avoid the
inconsistent
reads of *pmd for if-statements, which causes problem when both if-condition
reads *pmd and
the statements inside "if" reads *pmd again and two reads can give different
values.
Am I right about this?
If yes, the problem can be solved by something like:
if (!pmd_present(tmpval = *pmd)) {
check tmpval instead of *pmd;
}
Right?
I just wonder if we need some general code for all race checks.
Thanks.
--
Best Regards
Yan Zi
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