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    SubjectRe: the "read" syscall sees partial effects of the "write" syscall
    On Fri 18-09-20 08:25:28, Mikulas Patocka wrote:
    > I'd like to ask about this problem: when we write to a file, the kernel
    > takes the write inode lock. When we read from a file, no lock is taken -
    > thus the read syscall can read data that are halfway modified by the write
    > syscall.
    >
    > The standard specifies the effects of the write syscall are atomic - see
    > this:
    > https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09_07

    Yes, but no Linux filesystem (except for XFS AFAIK) follows the POSIX spec
    in this regard. Mostly because the mixed read-write performance sucks when
    you follow it (not that it would absolutely have to suck - you can use
    clever locking with range locks but nobody does it currently). In practice,
    the read-write atomicity works on Linux only on per-page basis for buffered
    IO.

    Honza

    --
    Jan Kara <jack@suse.com>
    SUSE Labs, CR

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