• Srivatsa S. Bhat's avatar
    PM / Sleep: Fix freezer failures due to racy usermodehelper_is_disabled() · b298d289
    Srivatsa S. Bhat authored
    Commit a144c6a6 (PM: Print a warning if firmware is requested when tasks
    are frozen) introduced usermodehelper_is_disabled() to warn and exit
    immediately if firmware is requested when usermodehelpers are disabled.
    
    However, it is racy. Consider the following scenario, currently used in
    drivers/base/firmware_class.c:
    
    ...
    if (usermodehelper_is_disabled())
            goto out;
    
    /* Do actual work */
    ...
    
    out:
            return err;
    
    Nothing prevents someone from disabling usermodehelpers just after the check
    in the 'if' condition, which means that it is quite possible to try doing the
    "actual work" with usermodehelpers disabled, leading to undesirable
    consequences.
    
    In particular, this race condition in _request_firmware() causes task freezing
    failures whenever suspend/hibernation is in progress because, it wrongly waits
    to get the firmware/microcode image from userspace when actually the
    usermodehelpers are disabled or userspace has been frozen.
    Some of the example scenarios that cause freezing failures due to this race
    are those that depend on userspace via request_firmware(), such as x86
    microcode module initialization and microcode image reload.
    
    Previous discussions about this issue can be found at:
    http://thread.gmane.org/gmane.linux.kernel/1198291/focus=1200591
    
    This patch adds proper synchronization to fix this issue.
    
    It is to be noted that this patchset fixes the freezing failures but doesn't
    remove the warnings. IOW, it does not attempt to add explicit synchronization
    to x86 microcode driver to avoid requesting microcode image at inopportune
    moments. Because, the warnings were introduced to highlight such cases, in the
    first place. And we need not silence the warnings, since we take care of the
    *real* problem (freezing failure) and hence, after that, the warnings are
    pretty harmless anyway.
    Signed-off-by: default avatarSrivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    Signed-off-by: default avatarRafael J. Wysocki <rjw@sisk.pl>
    b298d289
kmod.h 3.88 KB