# git rev-parse -q --verify ad7b4e8022b9864c075fe71e1328b1d25cad82f6^{commit}
ad7b4e8022b9864c075fe71e1328b1d25cad82f6
already have revision, skipping fetch
# git checkout -q -f -B kisskb ad7b4e8022b9864c075fe71e1328b1d25cad82f6
# git clean -qxdf
# < git log -1
# commit ad7b4e8022b9864c075fe71e1328b1d25cad82f6
# Author: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
# Date:   Tue Apr 3 15:54:02 2018 +0200
# 
#     cxl: Fix possible deadlock when processing page faults from cxllib
#     
#     cxllib_handle_fault() is called by an external driver when it needs to
#     have the host resolve page faults for a buffer. The buffer can cover
#     several pages and VMAs. The function iterates over all the pages used
#     by the buffer, based on the page size of the VMA.
#     
#     To ensure some stability while processing the faults, the thread T1
#     grabs the mm->mmap_sem semaphore with read access (R1). However, when
#     processing a page fault for a single page, one of the underlying
#     functions, copro_handle_mm_fault(), also grabs the same semaphore with
#     read access (R2). So the thread T1 takes the semaphore twice.
#     
#     If another thread T2 tries to access the semaphore in write mode W1
#     (say, because it wants to allocate memory and calls 'brk'), then that
#     thread T2 will have to wait because there's a reader (R1). If the
#     thread T1 is processing a new page at that time, it won't get an
#     automatic grant at R2, because there's now a writer thread
#     waiting (T2). And we have a deadlock.
#     
#     The timeline is:
#     1. thread T1 owns the semaphore with read access R1
#     2. thread T2 requests write access W1 and waits
#     3. thread T1 requests read access R2 and waits
#     
#     The fix is for the thread T1 to release the semaphore R1 once it got
#     the information it needs from the current VMA. The address space/VMAs
#     could evolve while T1 iterates over the full buffer, but in the
#     unlikely case where T1 misses a page, the external driver will raise a
#     new page fault when retrying the memory access.
#     
#     Fixes: 3ced8d730063 ("cxl: Export library to support IBM XSL")
#     Cc: stable@vger.kernel.org # 4.13+
#     Signed-off-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
#     Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
# < /opt/cross/kisskb/gcc-4.6.3-nolibc/powerpc-linux/bin/powerpc-linux-gcc --version
# < git log --format=%s --max-count=1 ad7b4e8022b9864c075fe71e1328b1d25cad82f6
# < make -s -j 48 ARCH=powerpc O=/kisskb/build/powerpc-next_44x_currituck_defconfig_powerpc CROSS_COMPILE=/opt/cross/kisskb/gcc-4.6.3-nolibc/powerpc-linux/bin/powerpc-linux-  44x/currituck_defconfig
# make -s -j 48 ARCH=powerpc O=/kisskb/build/powerpc-next_44x_currituck_defconfig_powerpc CROSS_COMPILE=/opt/cross/kisskb/gcc-4.6.3-nolibc/powerpc-linux/bin/powerpc-linux-  
/kisskb/src/kernel/printk/printk.c: In function 'devkmsg_sysctl_set_loglvl':
/kisskb/src/kernel/printk/printk.c:183:16: warning: 'old' may be used uninitialized in this function [-Wuninitialized]
/kisskb/src/kernel/rcu/srcutree.c: In function 'init_srcu_struct_fields':
/kisskb/src/kernel/rcu/srcutree.c:146:32: warning: 'levelspread[<U1e60>]' may be used uninitialized in this function [-Wuninitialized]
/kisskb/src/kernel/rcu/srcutree.c:94:6: note: 'levelspread[<U1e60>]' was declared here
/kisskb/src/drivers/tty/serial/8250/8250_core.c: In function 'univ8250_release_irq':
/kisskb/src/drivers/tty/serial/8250/8250_core.c:251:18: warning: 'i' may be used uninitialized in this function [-Wuninitialized]
/kisskb/src/drivers/tty/serial/8250/8250_core.c:231:19: note: 'i' was declared here
WARNING: modpost: Found 1 section mismatch(es).
To see full details build your kernel with:
'make CONFIG_DEBUG_SECTION_MISMATCH=y'
Completed OK
# rm -rf /kisskb/build/powerpc-next_44x_currituck_defconfig_powerpc
# Build took: 0:00:59.835094