Massive ISR to DSR delay caused by free()

["Peter Graf" <p dot graf at itknet dot de>]

Hi,

under rare circumstances, we experienced a massive timing overflow of a cyclical DSR. It turned out to be an issue with JFFS2, occurring after a system reboot if  long files had been written to the filesystem before. 

I found that the issue was only caused by JFFS2's 

malloc-ecos.c: jffs2_free_full_dnode() 

By using an oscilloscope, I could see that after the ISR was served, the DSR was delayed, just because free() was called by  jffs2_free_full_dnode(). More precisely, the CPU time was spent in memalloc's 

mvarimpl.inl: Cyg_Mempool_Variable_Implementation::insert_free_block()

At first I thought it was a locking issue within JFFS2. But in the end I could not see any scheduler locks which might have caused it.

I now have indications, that it could be a general issue with free(). A call to free() from even a low priority thread seems to delay DSR execution after the ISR!

The allocator is compiled to be thread-safe, but in my opinion it would be a major drawback if even a low priority thread could not call free() without delaying a critical DSR.

Can someone confirm this behaviour? Is it intended?

Many thanks,
Peter


 

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