Is this a bug that n_tty_set_room not serialized? If so please correctit.

[snakky . zhang]

Hi experts,

Seems there is an bug related to function n_tty_set_roomin source file
drivers/tty/n_tty.c. This function is used to set receive_room based on
the read_cntby both the consumer and the producer of the tty's read buffer.

But this function is not serialized, so I am afraid there is
a risk like: The producer make the buffer full and then call this 
function to
set the receive_room to 0; while it is preempted by the consumer that empty
the buffer(read_cnt is 0)and then call this very function to set the
receive_room to a non-zero value. But when the producer continue, 
receive_room
was set to 0 again. Thus both read_cnt and receive_room is 0.For consumer
theread_cnt is 0 that no data can be read; while for the producer, 
thebuffer
is full since the receive_room is 0. Both of them blockhere...

Two methods to avoid it:
A) Addor find a lock to serialize this function;
B) Once the producer find the buffer is full, double check/set the 
buffer with
n_tty_set_room. For example in function "flush_to_ldisc".

Please correct me if I misunderstand something. Or please fix it.

Thanks
Xiao
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