# 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-5.3.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc --version
# < git log --format=%s --max-count=1 ad7b4e8022b9864c075fe71e1328b1d25cad82f6
# < make -s -j 120 ARCH=powerpc O=/kisskb/build/powerpc-next_85xx_socrates_defconfig_powerpc-5.3 CROSS_COMPILE=/opt/cross/kisskb/gcc-5.3.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  85xx/socrates_defconfig
# make -s -j 120 ARCH=powerpc O=/kisskb/build/powerpc-next_85xx_socrates_defconfig_powerpc-5.3 CROSS_COMPILE=/opt/cross/kisskb/gcc-5.3.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  
INFO: Uncompressed kernel (size 0x57d9e0) overlaps the address of the wrapper(0x400000)
INFO: Fixing the link_address of wrapper to (0x600000)
Image Name:   Linux-4.16.0-rc4-gad7b4e8
Created:      Wed Apr  4 23:43:05 2018
Image Type:   PowerPC Linux Kernel Image (gzip compressed)
Data Size:    2841248 Bytes = 2774.66 kB = 2.71 MB
Load Address: 00000000
Entry Point:  00000000
Completed OK
# rm -rf /kisskb/build/powerpc-next_85xx_socrates_defconfig_powerpc-5.3
# Build took: 0:00:25.550527