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    SubjectRe: [PATCH] kmsg: properly handle concurrent non-blocking read() from /proc/kmsg
    >>> Kay Sievers <> 07/03/12 8:17 PM >>>
    >From: Kay Sievers <>
    >Subject: kmsg: properly handle concurrent non-blocking read() from /proc/kmsg
    >The /proc/kmsg read() interface is internally simply wired up to a sequence
    >of syslog() syscalls, which might are racy between their checks and actions,
    >regarding concurrency.
    >In the (very uncommon) case of concurrent readers of /dev/kmsg, relying on
    >usual O_NONBLOCK behavior, the recently introduced mutex might block an
    >O_NONBLOCK reader in read(), when poll() returns for it, but another process
    >has already read the data in the meantime. We've seen that while running
    >artificial test setups and tools that "fight" about /proc/kmsg data.
    >This restores the original /proc/kmsg behavior, where in case of concurrent
    >read()s, poll() might wake up but the read() syscall will just return 0 to
    >the caller, while another process has "stolen" the data.
    >This is in the general case not the expected behavior, but it is the exact
    >same one, that can easily be triggered with a 3.4 kernel, and some tools
    >might just rely on it.
    >The mutex is not needed, the original integrity issue which introduced it,
    >is in the meantime covered by:
    >"fill buffer with more than a single message for SYSLOG_ACTION_READ"
    >Cc: Yuanhan Liu <>
    >Cc: Jan Beulich <>
    >Signed-off-by: Kay Sievers <>

    This being a revert of all but a single change of
    4a77a5a06ec66ed05199b301e7c25f42f979afdc, which I had suggested to
    be reverted anyway:

    Acked-by: Jan Beulich <>

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