VM Synchronization Bug

[omegaphoenix]

I think the bug that sometimes breaks the parallel tests which test eviction has to do with accessing a frame while it is being deleted.

Currently the tests are failing with exit(-1) because !user_vaddr(fault_addr) is true in the exception.c page_fault() handler. This is caused by a fault within the kernel code. The kernel code that is being called is fetch_data_to_frame(). Here is the backtrace right before it fails:

#0 page_fault (f=0xc0132e64) at ../../userprog/exception.c:149
#1 0xc0022817 in intr_handler (frame=0xc0132e64) at ../../threads/interrupt.c:330
#2 0xc0022a1f in intr_entry () at ../../threads/intr-stubs.S:37
#3 0xc0132e64 in ?? ()
#4 0xffffffff in ?? ()
#5 0x00000008 in ?? ()
#6 0x00000282 in ?? ()
#7 0xc0111000 in ?? ()
#8 0xc003024d in fetch_data_to_frame (page=page@entry=0xc0136b8c) at ../../vm/page.c:227
#9 0xc002c919 in page_fault (f=0xc0132fb0) at ../../userprog/exception.c:160
#10 0xc0022817 in intr_handler (frame=0xc0132fb0) at ../../threads/interrupt.c:330
#11 0xc0022a1f in intr_entry () at ../../threads/intr-stubs.S:37
#12 0xc0132fb0 in ?? ()
#13 0x08048141 in ?? ()

The line in page.c that it is failing on is provided below:
struct frame_table_entry *fte = get_frame();

[omegaphoenix]

The issue can be reproduced by running the page-parallel test multiple times on qemu.

[omegaphoenix]

I think the bug is being caused by the frame being pinned for use after it is chosen for eviction but before it is evicted.

[omegaphoenix]

Kernel PANIC at ../../vm/frame.c:244 in free_frame(): assertion `fte->pin_count == 0' failed.

0xc002984c: debug_panic (.../../lib/kernel/debug.c:38)
0xc003016d: free_frame (...uild/../../vm/frame.c:246)
0xc0030447: evict (...uild/../../vm/frame.c:162)
0xc0030ba5: fetch_data_to_frame (...build/../../vm/page.c:227)
0xc002d4ca: sys_read (...../userprog/syscall.c:369)
0xc002da29: syscall_handler (...../userprog/syscall.c:124)
0xc0022817: intr_handler (..../threads/interrupt.c:330)
0xc0022a1f: intr_entry (threads/intr-stubs.S:38)
0x0804a22c: (unknown)
0x08048105: (unknown)
0x08048768: (unknown